The user input means of an electronic device can be implemented in various ways.
One approach for detecting a user input is generating an electrical field and detecting a deformation of the electric field by a user.
The electric field can be associated for instance to the area of a touch-screen. The caused deformation will then depend on the position at which the touch-screen is touched by some object, for instance by a finger of the user. The touched position can thus be determined by evaluating the deformation.
For generating and monitoring such an electrical field, different sensor technologies can be employed. They may be based for instance on a capacitive detector or on a resistive-capacitive detector.
A capacitive detector comprises at least one conductive plate, which forms a capacitance with at least one other conductive plate or an object. With a pure capacitive detector, an electric field is induced and disturbances of the electric field by a user can be detected for instance by monitoring the capacitance values of four capacitors at the four corners of a monitored area. The capacitance values can be used for detecting whether there is some object in close vicinity of the detector or not, and at which position. A grounding around the capacitive detector could be used to reduce the interference by other capacitances in the vicinity and to limit lateral interactions of the capacitive detector.
A Resistive-capacitive detector is a homogeneous or non-homogeneous electrically resistive element, for example a square resistance of 1-5 kOhm. It can be made for instance of a graphite paper, or of a plastic foil with a conductive, transparent indium tin oxide (ITO) layer. With a resistive-capacitive detector, again an electric field is induced. In this case, however, the varying resistance is used in a position calculation. Changes in the resistance can be detected as changes in an electrical current. A resistive-capacitive detector can be used for instance for realizing sliders, touchpads, keypads, etc.
User input technologies that are based on a deformation of an electric field in free air can be realized in a robust manner at low costs. However, their usability is limited, because the detection of a deformation is rather sensitive.
The field strength is not equally distributed over a monitored area. Therefore, sensitivity and resolution are different in different areas of the field source, which makes it difficult to calibrate the sensor for enabling a calculation of the correct position. Moreover, an electric field that is based on a strong source is far reaching and the lines of force of the field are bended. Therefore, an object will cause a deformation of the field already when approaching a desired position of a monitored area, and the signal will change only slowly with increasing proximity. This makes it is difficult to set a threshold level for the detection of a touch. Also different finger sizes and gloves put demands on setting the threshold level of the sensor.
These problems are also of relevance in case a single large sensor, like a large resistive-capacitive detector, is used for several keys of a keypad. The free field form complicates the algorithm that is used for determining the key that a user presses based on a detected deformation. If a user is not concentrated in a proper action of pressing a key, a false key may be indicated, which reduces the usability of the keypad significantly.